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discharge capacity utilization (i.e., 0.37 Ah) is reached to avoid effects of non-linear fade at later cycles. The steeper initial capacity drop may be a consequence of diminishing activity of the oxygen groups on the electrode formed during oxidative pretreatment that occurs as the electrode undergoes prolonged cycling, whose deactivation is more pronounced towards the Cr redox reaction due to its lower potential [46]. The untreated electrolyte reaches the cutoff within 17 cycles (13.9 h total duration), while the purified electrolyte lasts a prolonged 87 cycles (71.7 h total duration) prior to reaching the same capacity retention. This corresponds to decay rates of 2.94 % / cycle and 0.57 % / cycle for the first 50 cycles for the unpurified and purified electrolytes, respectively. We note that our own efforts to rebalance spent electrolytes by mixing used posolyte and negolyte together, dividing the mixed electrolyte into two equal volumes, and resuming operation did not lead to significant capacity recovery, suggesting that the mechanisms of capacity fade were not solely due to crossover. The same purification protocol was performed with 10× the original electrolyte volume to evaluate its effectiveness as a function of volume to be purified. The same total electrolyte volume of 50 mL was taken from the larger volume of purified electrolyte and cycled. Figure V-3e shows that while capacity fade was mitigated compared to no treatment, it is more rapid than with a smaller volume of purified electrolyte. One possible explanation is that not all the electrolyte impurities are removed with the larger volume of electrolyte if purified using the same electrode size (i.e., there is not enough electrode surface area to plate out all the metal impurities present in the larger electrolyte volume). This hypothesis implies there is, perhaps, a ratio of electrolyte volume to electrode surface area that cannot be exceeded for sufficient purification or operation; quantification and optimization of such a ratio should be the focus of future work. We seek to contextualize our results within the broader efforts towards alleviating capacity fade in Fe-Cr RFBs. Summaries of performance metrics from a non-exhaustive list of previously reported literature is summarized in Table V-1. Some of the data is adapted in part from the recent review paper by Sun and Zhang [204]. Approximate averages for the coulombic, voltaic, and energy efficiencies of longer-duration galvanostatic cycling tests are shown, along with estimated discharge capacity decay per cycle. Self-reported data were used whenever possible; if the capacity decay rate was not reported, the decay rate to 50% of the original capacity was extracted from published figures. The use of different flow field designs, electrode materials and thicknesses, electrolyte compositions, in-house cell architectures, laboratory practices, and cycle numbers 98PDF Image | Bringing Redox Flow Batteries to the Grid
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